1. Field of the Invention
The present invention concerns surgical implants, particularly bone screws.
2. Discussion of the Known Art
U.S. Pat. No. 5,474,558 (Dec. 12, 1995) discloses a procedure and system for spinal pedicle screw insertion, wherein pedicle screws can be inserted into vertebrae of the spine in a way that reduces the likelihood of nerve damage due to improper screw placement. See also U.S. Pat. No. 5,196,015 (Mar. 23, 1993). Basically, as a screw opening is formed in the pedicle, an electric potential is applied to the inside wall of the opening while the patient is observed for a nervous reaction such as, e.g., leg twitching. Formation of the opening is continued while the electric potential is applied, until a desired hole depth is obtained without monitoring a nervous reaction. If a reaction occurs, the direction in which the opening was then being formed, is changed. The mentioned '558 and '015 U.S. patents, and U.S. Patent Application Pub. No. 2006/0173374 (Aug. 3, 2006), disclose a system and tools for carrying out the foregoing procedure.
Conventional pedicle screws available from, e.g., Biomet, Inc., of Warsaw, Ind., and SpineUniverse, LLC, of Montclair, N.J., are made from titanium, stainless steel, or other strong medical grade alloy. The screws are typically provided with so-called polyaxial or mobile cup-shaped head extensions for seating and locking associated rods once the screws are properly located and driven into the spine. The screw heads usually have hexagonal or slotted recesses for receiving a matching drive tool bit.
An expandable screw for spine fixation offered by Biomet, Inc., under the mark Biomet® Omega21™, is cannulated to accept an expansion peg. A lower (distal) threaded portion of the screw body is formed into four quadrants or fins that expand radially outward to lock the screw in bone tissue when the peg is inserted through a central passage in the screw. Withdrawing the peg allows the fins to collapse for easy removal of the screw when necessary, according to the manufacturer.
Other cannulated screws available from Synthes®, Inc., of West Chester, Pa. and identified as Pangea® pedicle screws, have central bores that are open at both ends of the screws. To insert a screw into bone, the tip of a guide (Kirschner) wire is first seated at a certain depth in a spinal pedicle using a cannulated awl. The awl is removed, and a pedicle probe is guided over wire to prepare a screw channel. After an optional channel threading step, the free end of the wire is inserted through the bore of a selected pedicle screw, and the screw is driven 2-3 rotations into the bone using a cannulated screwdriver shaft. The wire is then removed, and the screw driven further to a desired depth in the bone. A tutorial on use of the Synthes cannulated screws is available on the Internet at <http://www.synthes.com/site/fileadmin/Shared/shop/Printed_Materials/Techique_Guide/Spine—2007—08/036.000.941.pdf>.
U.S. Pat. No. 7,218,232 (May 15, 2007) discloses a bone fastener having a threaded engaging portion and a head portion. In one embodiment, the fastener has a cavity formed in the head portion, and a storage device such as a RFID tag is embedded in the cavity. According to the patent, the device may carry information that is specific to both the fastener and the patient.
There remains a need for a bone screw that is constructed to cooperate with the earlier mentioned systems and tools, and thus reduce substantially the possibility of nerve damage resulting from an improperly placed screw.